The power factor is the ratio of real power to apparent power in a transmission and distribution system. It is used by the power industry to gauge the efficiency of a power distribution system. Real power measures the ability of an electrical load to perform work in a set time, and is associated with power consumption by a resistive load. Reactive power is associated with power generated by the impedance of a reactive load, which can be inductive or capacitive. The apparent power is the sum of the real power and the reactive power and may be equal to or greater than the real power depending on the parasitic reactive power.
In a purely resistive circuit, voltage and current waveforms are in phase and the power factor is one. In a circuit having reactive loads such as capacitive or inductive loads, there can be a phase difference between the current and voltage waveforms, which causes the power factor to be less than one. Because the stored energy returns to its source and does not perform work at the load, a circuit with a lower power factor will receive higher currents for a given quantity of received real power than a circuit with a higher power factor.
Power distribution networks with a low power factor are undesirable, because they require power lines to carry more current than necessary to provide real power to customers. The additional current can result in line losses, reduced life span of the equipment and the need to build new power generating facilities.
According to one study, the average power factor for a residential consumer is 0.87. This means that 87% of the current contributes to real power and 13% is used for reactive power. The power factor can decrease drastically during peak demand hours, when reactive power takes up a larger percentage for home appliances such as HVAC motors, washers. This can cause power distribution issues.
Power utilities have made efforts to improve the power factor of their networks, by adding capacitor banks and other components at substations before the secondary or distribution lines (usually 11-2.4 kV) that distribute power to the consumers. These other components include phase-shifting transformers, static VAR compensators, and flexible AC transmission systems (FACTS) to control reactive power flow for reduction of losses and stabilization of system voltage. However, these capacitor banks and other components do not eliminate the majority of the power losses. Many of these components and capacitors are also large, difficult to install, suffer from transient breakdown, require extra infrastructure, and need expensive on-going maintenance.
Another approach used by utilities to improve the power factor is to place a static capacitor bank in the secondary or distribution lines (also called radial lines or feeder lines). However, a static capacitor bank has no control capabilities, and a capacitance constantly on-line can be detrimental to the distributing system, by causing higher line current at periods of low usage by the consumers.
Accordingly, there is a need for improved systems for correcting the power factor in power transmission and distribution systems, which do not have at least some of the disadvantages associated with known systems.
In addition, severe adverse conditions can arise in transmission systems. These conditions may require load shedding. Such conditions include and are not limited to extreme demand, generation rejection, and transmission station/line loss. When such conditions arise, transmitters will systematically drop feeder lines that supply distribution to the local areas, to maintain the system's integrity.
The dropping of a feeder line involves the total disruption (i.e. loss) of electricity delivery for every consumer supplied by distribution stations using that feeder. This is very disruptive to residential and commercial customers who are connected to this service.
In the case of a widespread blackout, like that which occurred in north-eastern North America in 2003, customers using non-critical appliances that draw significant power, slowed the reinstatement of the grid. The shock of turning on all of the load on a radial line or feeder, including sedentary (i.e. switched on) air conditioners was a significant challenge. There presently exists no means for grid operators to decrease loading to avoid widespread load shedding or to minimize startup load for reconnecting the customers to the grid.
Reactive power is used for the transmission of electricity by supporting the transactions of power from one bus to the other. Consequently this reactive power also provides voltage support. This reactive power is usually generated by power stations or capacitor banks at key transmission busses. The reactive draw from customers on distribution feeders attached to the transmission busses could be used to assist in transaction and voltage support but this has never been accomplished as yet.